1. Field of the Invention
This invention relates to a pneumatic tire, and more particularly to a pneumatic radial tire belonging to a category known as so-called run-flat tire and capable of running over a given distance at a state that an internal pressure becomes zero (atmospheric pressure) or near to zero due to puncture or the like. Preferably, the invention relates to a pneumatic radial tire having a ratio of section height to section width or an aspect ratio of not less than 60 and an excellent run-flat durability (xe2x80x9crun-flatxe2x80x9d means the running at a punctured state) when being run at a high speed over a relatively long distance at the punctured state while holding a low cost and a handling easiness.
2. Description of Related Art
The run-flat type radial tire (hereinafter abbreviated as run-flat tire) is mainly used in vehicles such as passenger cars and the like under a relatively small loading applied to the tire. It is demanded that when the tire becomes a flat (punctured) state, even if it suddenly occurs during not only the running on general road but also the high-speed running on an expressway, the run-flat tire can safely be run without considerably degrading the steering stability of the vehicle, particularly a passenger car, and even if the running is continued at the punctured state over the relatively long distance, the run-flat tire can be safely and surely run over a given distance, for example, 80-160 km to an intended place without detaching from a rim used (approved rim) and causing the tire breakage.
For this end, there are proposed run-flat tires having various structures, which are often combined with a contrived approved rim. The tires for such proposals are roughly classified into a super low-section profile tire having an aspect ratio of less than 60 and a low-section profile tire having an aspect ratio of 60-80 in which the section height is relatively high.
As to the super low-section profile tire, an example (20) of run-flat tires put into practical use in the market at most is shown in FIG. 5. This tire 20 is a tire having a structure that a thick rubber reinforcing layer 9 having a crescent shape in section is arranged at an inner surface side of an innermost carcass ply 6-1 over a zone ranging from a bead portion 2 through a sidewall portion 3 to an end of a tread portion 4. Such a tire is often mounted onto a vehicle premising high-speed running such as a sports car, a sports-type car or the like.
In order to mitigate a degree of collapse deformation as far as possible when the tire 20 having the thick rubber reinforcing layer 9 is run at a flat state under loading, a radial carcass 6 is constituted by two or more plies comprising a turnup ply 6-1 wound around a bead core 5 from an inside of the tire toward an outside thereof and a down ply 6-2 enveloping the turnup ply 6-1 from the outside thereof, and a hard stiffener rubber 8 is arranged so as to extend from an outer peripheral surface of the bead core 5 up to a position near to a maximum tire width, and as the case may be, a rubberized layer of Kevlar cords or steel cords (which is called as an insert ply) is disposed in a zone ranging from the bead portion 2 to the sidewall portion 3.
On the other hand, the low-section profile tire having a relatively high section height is recently and frequently mounted onto an imported or domestic high-grade passenger car having a relatively great displacement. This type of the tire is possible to be run at the run-flat state in cooperation with a rim to be assembled, wherein an incorporation of a protect member for pushing a bead portion of the tire toward a rim flange onto the rim is the main current. Because, a height of a sidewall portion 3 in the low-section profile tire (see FIG. 5) is considerably higher than that of the super low-section profile tire and the sidewall portion 3 violently bends by a height difference between both the tires and as a result the desired run-flat durability can not be obtained.
In the tire 20 having the above construction as shown in FIG. 5, it is proposed and practiced to increase the thickness or the height of the rubber reinforcing layer 9, or to largely increase the hardness or modulus of rubber itself for improving the durability in the run-flat running. However, they have a limitation on manufacture and a limitation on the rising width of the cost, so that when the internal pressure is suddenly at a state of zero, it is particularly insufficient to ensure the steering stability of the vehicle in the high-speed running and also the durability is insufficient in the continuation of the running at the run-flat state. Therefore, it is desired to develop tires holding the above performances and improving the run-flat durability to a level causing no problem in practical use.
In the case of incorporating the protect member onto the rim, there is firstly a problem in so-called rim assembling operability because it is not easy to assemble the tire onto a wheel. Secondly, it is unavoidable to largely increase the weight in the tire-wheel assembly, which brings about significant increase of unsprung mass in the vehicle and hence the ride comfort of the vehicle against vibrations is considerably degraded, so that it is unsuitable as the tire for use in a high-grade passenger car.
The inventors have thoroughly investigated a position and a cause of tire trouble generated in time series during the run-flat running of the tire 20 shown in FIG. 5 with respect to the run-flat durability and confirmed that a position of trouble most quickly generated in the tire is an end portion of xcex3-region in the belt 7 (see FIG. 5) and then such a trouble successively causes a trouble of the rubber reinforcing layer 9 in xcex2-region of the sidewall portion 3 (see FIG. 5) and a trouble of the stiffener rubber 8 in xcex1-region of the bead portion 2 (see FIG. 5). Therefore, the inventors have already proposed a pneumatic tire 21A having particularly an aspect ratio of not less than 60, in which a cushion rubber 10 as shown in FIG. 6 is interposed between the end portion of the belt 7 and the outermost carcass ply 6-2 (see FIG. 5) and tan xcex4 of the cushion rubber 10 is made not more than that of a coating rubber for the cords of the carcass ply 6 as described in Japanese Patent Application No. 10-121422. This tire 21A is demonstrated that the remarkable improvement of the durability can advantageously be realized in the end portion of the belt 7 during the run-flat running to attain an excellent run-flat durability.
However, it is demanded to further improve the run-flat durability without stop at the present time. The run-flat durability satisfying the above demand is conditioned to satisfy that it is possible to at least run over a distance of 80-160 km as previously mentioned under the continuation of running at an upper level of limit speed at the run-flat state on the expressway even in the pneumatic tire having the aspect ratio of not less than 60. Under such a severer condition, it is the present state that it is required to more further improve the run-flat durability improved in the aforementioned pneumatic tire 21A.
It is, therefore, an object of the invention to provide a pneumatic tire having particularly an aspect ratio of not less than 60 which controls the cost to a level not causing trouble in practice, holds the handling easiness for rim assembling operability or the like and maintains the ride comfort within an acceptable range without causing inconvenience in the production applying the conventional thick rubber reinforcing layer, and guarantees the safe running of the vehicle such as passenger car or the like even if air is rapidly released due to the puncture or the like, and does not cause fatal trouble even if the high-speed running on the expressway is continued at a limit speed or a speed near thereto for a long time, and can develop a highly excellent run-flat durability.
According to the invention, there is the provision of a pneumatic tire comprising a radial carcass extending between a pair of bead cores embedded in the respective bead portions to reinforce a pair of sidewall portions and a tread portion and comprised of one or more rubberized cord plies, a belt arranged on an outer peripheral surface of the carcass to reinforce the tread portion, a bead filler rubber taperingly extending from a position just above the bead core toward an end of the tread portion, and a rubber reinforcing layer arranged at an inner surface side of an innermost carcass ply from a position near to the bead core in the bead portion to a position near to the end of the tread portion and having substantially a crescent shape at section thereof, in which at least one rubber protection sheet being relatively soft is disposed between the bead filler rubber and the carcass ply surrounding it and/or between the rubber reinforcing layer and the carcass ply adjacent thereto.
In a preferable embodiment of the invention, at least one ply of the carcass is a turnup ply wound around the bead core from an inside of the tire toward an outside thereof, which consists of a toroidally extending main body and a turnup portion.
Moreover, when the carcass is composed of two or more plies, it has so-called up-down ply structure consisting of a turnup ply having a turnup portion as mentioned above and a down ply enveloping the turnup ply inclusive of the turnup portion from the outside.
In another preferable embodiment of the invention, in a radial section of a tire-rim assembly when the tire is mounted onto a recommended rim and inflated under a pressure corresponding to 15% of a maximum air pressure, the rubber protection sheet is existent over both sides of a straight line drawn from a curvature center of a flange of the recommended rim at an inclination angle of 60xc2x0 outwardly in a radial direction of the tire with respect to a line segment drawn from the curvature center in parallel to a rotating axial line of the tire toward the inside of the tire.
The term xe2x80x9cmaximum air pressurexe2x80x9d used herein means a maximum air pressure (air pressure corresponding to a maximum load) disclosed in xe2x80x9cGeneral Informationxe2x80x9d of xe2x80x9cThe Tire and Rim Association Inc.xe2x80x9d (hereinafter abbreviated as xe2x80x9cTRA standardsxe2x80x9d) and the term xe2x80x9crecommended rimxe2x80x9d used herein means a recommended rim disclosed therein. More particularly, as the maximum air pressure and the recommended rim are used numerical values and rim sizes disclosed in xe2x80x9cLOAD/INFLATION PRESSURE TABLExe2x80x9d and xe2x80x9cRIM CONTOURS TABLExe2x80x9d established by every kind of tires in the TRA standards. Moreover, the recommended rim is a rim underlined in the lists of xe2x80x9cRIM CONTOURS TABLExe2x80x9d.
Furthermore, the wordings xe2x80x9cthe tire is mounted on the recommended rim and inflated under an air pressure corresponding to 15% of the maximum air pressurexe2x80x9d used herein means that the tire mounted on the recommended rim is once inflated to an air pressure no less than the maximum air pressure to sufficiently fit the tire to the recommended rim and thereafter the filled air is deflated up to a weak pressure corresponding to 15% of the maximum air pressure, or once the filled air is deflated up to an internal pressure of zero, the compressed air is again filled to a weak pressure corresponding to 15% of the maximum air pressure while holding the sufficient fitted state.
In the other preferable embodiment of the invention, the rubber protection sheet is existent between line segments in parallel to the rotating axial line of the tire respectively passing through an outer end of the bead filler rubber in the radial direction of the tire and an inner end of the rubber reinforcing layer in the radial direction of the tire.
In a still further preferable embodiment of the invention, when the rubber protection sheet is disposed along the turnup portion of the carcass ply between the turnup portion and the bead filler rubber, a height Ha of an outer end of the rubber protection sheet in the radial direction of the tire as measured from an outermost end of the bead core in the radial direction of the tire is not more than two times a height Hb of an intersecting point between the straight line drawn from a curvature center of a flange of the recommended rim at an inclination angle of 60xc2x0 outwardly in a radial direction of the tire with respect to a line segment drawn from the curvature center in parallel to a rotating axial line of the tire toward the inside of the tire and an outer surface of an outermost carcass ply as measured by the above same method.
As the properties of the rubber protection sheet, it is preferable that the rubber protection sheet has 50% modulus of 0.30-0.84 times that of the rubber reinforcing rubber and tan xcex4 at 25xc2x0 C. of 0.04-0.11.
The value of 50% modulus is determined by a calculating equation disclosed in the term of xe2x80x9ctensile stressxe2x80x9d according to the contents disclosed in xe2x80x9cTensile test method in the cured rubberxe2x80x9d of JIS K 6251-1993. Moreover, the value of tan xcex4 is determined by calculating a kind of the deformation as tensile according to xe2x80x9cA case of the waveshape in the load and the waveshape in the flexurexe2x80x9d among xe2x80x9c(1) non-resonating methodsxe2x80x9d disclosed in xe2x80x9cMethod for testing dynamic nature in the cured rubberxe2x80x9d of JIS K 6394-1995. Moreover, the value of tan xcex4 in the actual test is determined under test conditions that an initial tensile load is 160 gf, a dynamic strain is 1.0%, and a frequency is 52 Hz.
As the thickness of the rubber protection sheet, it is preferable that rubber protection sheet has a thickness of 0.4-4.0 mm. Strictly speaking, the rubber protection sheet is more preferable to have a thickness of 0.6-4.0 mm when it is disposed between the bead filler rubber and the main body of the carcass ply and/or between the rubber reinforcing layer and the main body of the carcass ply, and 0.4-4.0 mm when it is disposed along the turnup portion of the carcass ply between the turnup portion and the bead filler rubber.